arrow

GH-36157: [C++][Dev] Add support for using python3 to run IWYU (#36159)

### Rationale for this change

Python executable name is hard-corded in `cpp/build-support/iwyu/iwyu.sh`. If a developer uses `python3` for Python executable, the developer can't use the script.

### What changes are included in this PR?

Add support for customizing Python executable name (and path) by `PYTHON` environment variable like other scripts.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #36157

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>